1. Field of Invention
This invention generally relates to titanyl phthalocyanine (TiOPC) having a novel crystal structure, a process for producing the titanyl phthalocyanine and an electrophotographic photoreceptor containing the titanyl phthalocyanine. More specifically, the present invention relates to improved organic photoreceptors containing modified titanyl phthalocyanine as the charge generation material for use in electrophotographic devices such as copiers and laser printers.
2. Description of Related Art
Photoreceptor is the main component responsible for the formation of images in electrophotographic processes that take place in copiers and laser printers. There are two main types of photoreceptors: those containing inorganic photoconductive materials and those containing organic photoconductive materials. Inorganic photoconductive materials, such as selenium, silicon, and mixtures of arsenic and selenium, have been developed and used as photoreceptors in electrophotography. Although appreciable sensitivity and long life can be achieved, the inorganic photoconductive materials suffer from several major drawbacks, such as toxicity and high cost of production. Organic photoconductive materials, on the other hand, offer many attractive features that includes nontoxicity, relatively low cost, low pollution, broad spectral response ranging from visible to infrared light, and flexible choice for use. Stringent requirements are imposed on the photoconductive as well as mechanical properties of electrophotographic photoreceptors. Suitable candidate materials for use as photoreceptors are required to exhibit, not only efficient charge generation and charge transport properties, but also structural integrity and robustness so as to withstand mechanical abrasion during image development cycles. Most commercial copiers and laser printers nowadays have used function-separated photoreceptors to achieve the various requirements. General structures of photoreceptors include an aluminum substrate and sequentially forming an undercoating layer, a charge generation layer and a charge transport layer thereon by dip coating. Organic compounds such as azo, bisazo and perylene pigments are usually chosen as the charge generation material for photoreceptors that are sensitive to visible light. For printers that utilize semiconductor laser (LD) or light-emitting diodes (LED) as the light source, photoreceptors are required to be photosensitive to near-infrared beam in the wavelength ranges of 750 nm to 850 nm, particularly in the main peak wavelength of 780 nm. In this category, metal-free phthalocyanines, metallophthalocyanines and squrayliums have been found to exhibit substantial photosensitivity in the near-infrared ranges.
Among the infrared-sensitive organic materials, titanyl phthalocyanines are especially of interest because of their high efficiency of charge generation. The structure of titanyl phthalocyanine is shown in FIG. 1. There are many conventional technologies and prior art patents disclosed to develop methods that can cost-effectively mass produce the highly infrared-sensitive titanyl phthalocyanine, with consistent and further improved quality, for use as efficient electrophotographic photoreceptors. The efficiency of charge generation of titanyl phthalocyanine was found to be strongly dependent on its crystal structure. Most of the reported methods for modifying the crystal structure of titanyl phthalocyanines involve an initial acid-pasting treatment step, in which strong protonic acids such as sulfuric acid are used to dissolve the titanyl phthalocyanine material, and the resultant solution was subsequently precipitated in a nonsolvent such as water, to obtain amorphous powder of titanyl phthalocyanine. Another method for modifying the crystal structure of titanyl phthalocyanines utilizes the ball milling of the dry titanyl phthalocyanine material; this method was shown to have the same effect to obtain the amorphous phase titanyl phthalocyanine.
The essence of the methods reported in the art for the modification of the crystal structure of titanyl phthalocyanine lies in the subsequent solvent treatment of the amorphous titanyl phthalocyanine. Effective organic solvents include chlorobenzene, dichlorobenzene and dichloroethane, or ethers such as n-butyl ether and ethylene glycol n-butyl ether. In U.S. Pat. No. 5,059,355, it was disclosed a process for the preparation of crystalline titanyl phthalocyanine showing distinct diffraction peaks at Bragg angles (2.theta..+-.0.2.degree. )of 9.3, 10.6, 13.2, 15.1, 15.7, 16.1,20.8,23.3 and 26.3 degrees in the X-ray diffraction spectrum, which includes mechanically pulverizing titanyl phthalocyanine, dispersing it in water to form a suspension, adding an organic solvent into the suspension and heating the suspension. In U.S. Pat. No. 5, 132, 197, it was disclosed that highly photoconductive titanyl phthalocyanine having Bragg diffraction angles of 9.0, 14.2, 23.9 and 27.1 degrees can be produced by acid-pasting treatment to obtain amorphous titanyl phthalocyanine, then by treatment with methanol to obtain low crystalline titanyl phthalocyanine, followed by a wet milling in n-butyl ether or pinene or ethylene glycol n-butyl ether. The importance of the presence of water during the modification of crystal forms of titanyl phthalocyanine was further addressed in U.S. Pat. No. 5,298,617. It was found ammonia has the same effect as water that their molecules might have actually become associated with titanyl phthalocyanine after the treatment processes disclosed in U.S. Pat. No. 5,567,559. The ammonia-modified titanyl phthalocyanine has Bragg diffraction angles of 7.5, 9.5, 13.6, 14.3, 17.9, 24.0, 27.2 and 29.1 degrees.
Although highly photoconductive titanyl phthalocyanines have been reported in the art, the mechanism of crystal transformation and the role of different solvent media remained unclear in the above-mentioned prior art disclosures. Furthermore, the possibility of water doping on the titanyl phthalocyanine molecule cannot be neglected. It was also shown in many reports that the electric conductivity of titanyl phthalocyanine and other related phthalocyanines has increased after treatment with water since water, like oxygen, can oxidize the titanyl phthalocyanine material. Therefore, it can be expected that the dark decay may become a serious problem when water-treated titanyl phthalocyanine is used as the charge generation component of the photoreceptor. The adverse effect of substantial dark decay associated with the titanyl phthalocyanine produced by the prior art processes will cause undesirable foggy images to be produced in the printed copies. On the other hand, the prior art titanyl phthalocyanines generally show a higher distinct absorption peak which appears at the wavelength of 800 nm to 820 nm and a lower one around the main peak wavelength of 780 nm for the light source.